A conventional microstrip antenna comprises a ground plane and a radiator isolated therefrom by a dielectric layer. The resonance frequency of the microstrip antenna depends on the dimensions of the radiator and on the distances between the radiator and ground plane. Microstrip antenna constructions are described in general e.g. in the "Handbook of Microstrip Antennas" by J. R. James and P. S. Hall (Eds.), Vol 1, Peter Peregrinus Ltd, London 1989 and in "Analysis, Design, and Measurement of Small and Low-Profile Antennas" by K. Hirasawa and M. Haneishi, Artech House, Boston 1992. From the prior art it is known microstrip antenna constructions in which one edge of the radiator is short-circuited to the ground plane. Using such an arrangement a given resonance frequency can be achieved with considerably smaller physical dimensions than the simplest microstrip antenna described above. FIG. 1 illustrates such a microstrip antenna in cross section. FIG. 1 shows a ground plane 20, radiator 10 and a feed line 30. The radiator 10 is short-circuited at its first end to the ground plane 20 through a short-circuiting part 11. The second end of the radiator is open. FIG. 1 does not specifically show the dielectric medium which may be air, for example. Microstrip antennas are often implemented on printed circuit boards, in which case there is the usual dielectric pcb material between the radiator 10 and ground plane 20.
A typical problem with planar antenna constructions according to the prior art is their thickness and narrow band. Antennas used in personal mobile communications devices must be small in size. However, making the microstrip antenna thinner makes the usable frequency band of the antenna narrower. Many mobile communications systems require a relatively wide frequency band, e.g. the DCS-1800 system requires a 10% frequency band, approximately, relative to the center frequency.
In the GSM system, for example, the transmit and receive bands are spaced at 45 MHz from each other, the transmit band being 890-915 MHz and the receive band 935-960 MHz. With an antenna of a single resonance the frequency band should be considerably wide, at least 890-960 MHz in the case of GSM. Because of manufacturing tolerances and objects near the antenna, such as e.g. the hand of the user, which affect the resonance frequency, the bandwidth must be even wider than in the ideal case.
A second approach is to realize an antenna with two frequency bands such that the first frequency band corresponds to the transmit band and the second frequency band corresponds to the receive band. In that case the frequency bands of the antenna need not be as wide as those of a single-band antenna. Such dual-band antennas may comprise e.g. two helix antennas tuned to different frequencies or a combination of a rod antenna and a helix, where the rod and helix are tuned to different frequency ranges. Such constructions are described e.g. in Finnish patent application no. 952780. However, such helix antenna constructions are difficult to realize inside the housing of a mobile communications device. Furthermore, these arrangements only operate on two frequency bands. However, future multimode mobile communications devices operating in more than one mobile communications system require antenna constructions operating in more than two separate frequency bands.
Microstrip constructions can be used to realize many different antenna solutions, say, constructions with more than one operating band. FIG. 2 shows an example of such a construction. FIG. 2 shows a ground plane 20, radiator 10 and a feed line 30. A gap 15 divides the radiator 10 in two parts having different resonance frequencies. The radiator may also have more gaps and more parts in which case there are several resonance frequencies as well.
Planar dual-band antenna constructions are disclosed e.g. in US Pat. No. 5,124,733. Said patent publication discloses a microstrip antenna construction which has in addition to a ground plane one active radiating element and a second passive element. The elements are quarter-wave elements short-circuited to the ground plane through one edge. The elements have differing resonance frequencies so that the antenna construction has two separate operating frequency bands. A disadvantage of such a solution is the thickness of the two stacked antenna elements. Furthermore, this solution, too, allows for operation on two frequency bands only.
FIG. 2 shows only one feed line 30. It is also known to use more than one feed point and feed line so that the properties of the antenna, such as the resonance frequency, directivity and diversity characteristics, for instance, can be influenced by choosing the feed point used. The characteristics of the antenna construction can also be influenced by the shape and size of the radiator in the antenna construction and by the size difference and distance between the radiator and ground plane, for example.